1. Field of the Invention
This invention relates to a method and apparatus for detecting an energy level of residual noise on a stimulable phosphor sheet. This invention particularly relates to a method and apparatus for accurately detecting residual noise having a high energy level on a stimulable phosphor sheet in a radiation image read-out method and apparatus. This invention also relates to a method and apparatus for erasing residual noise, wherein the method and apparatus for detecting an energy level of residual noise on a stimulable phosphor sheet are utilized.
2. Description of the Prior Art
It has been proposed to use stimulable phosphors in radiation image recording and read-out systems. Specifically, a radiation image of an object, such as a human body, is recorded on a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet). The stimulable phosphor sheet, on which the radiation image has been stored, is then scanned with stimulating rays, such as a laser beam, which cause it to emit light in proportion to the amount of energy stored thereon during its exposure to the radiation. The light emitted by the stimulable phosphor sheet, upon stimulation thereof, is photoelectrically detected and converted into an electric image signal. Radiation image recording and reproducing systems have also been proposed which comprise the radiation image recording and read-out systems and radiation image reproducing systems for reproducing the radiation image of the object as a visible image on a recording material from the obtained image signal.
In the radiation image recording and read-out systems or the radiation image recording and reproducing systems described above (hereinbelow be referred to as the "radiation image recording and read-out systems, or the like"), energy stored on the stimulable phosphor sheet during its exposure to radiation is not completely released therefrom during the radiation image read-out operation. Therefore, after the radiation image has been read out from the stimulable phosphor sheet, energy remains on the stimulable phosphor sheet. The remaining energy is due to the radiation image stored on the stimulable phosphor sheet. If the remaining energy on the stimulable phosphor sheet is of a level which is not negligible, when a next radiation image is recorded on the stimulable phosphor sheet and reproduced, the remaining energy will be reproduced together with the radiation image and will cause noise to occur in the reproduced visible image. Therefore, after the radiation image has been read out from the stimulable phosphor sheet, energy (hereinbelow referred to as "noise") remaining on the stimulable phosphor sheet should be erased. In order for noise to be erased, the remaining energy may be released from the stimulable phosphor sheet. For this purpose, for example, the stimulable phosphor sheet may be exposed to the ordinary environmental light for a long time and may thereby be caused to release the remaining energy. Alternatively, the stimulable phosphor sheet may be exposed to erasing light, such as visible light, having a high intensity, and the remaining energy may thus be released quickly from the stimulable phosphor sheet. In this manner, erasing energy is given to the stimulable phosphor sheet in order to erase noise such that the stimulable phosphor sheet can then be reused for the image recording operation.
In the aforesaid radiation image recording and read-out systems, or the like, as disclosed in, for example, U.S. Pat. No. 4,584,482, the light emitted by a stimulable phosphor sheet and carrying the radiation image information stored thereon is detected, and the radiation image information is thus read out. The dose of radiation delivered to the stimulable phosphor sheet is then calculated from the read-out radiation image information. Also, the amount of energy of noise remaining on the stimulable phosphor sheet after being exposed to the stimulating rays is calculated from the calculated dose of radiation delivered to the stimulable phosphor sheet, and the amount of erasing energy required to erase noise is then calculated. Thereafter, an erasing operation is carried out by irradiating erasing light having a predetermined intensity to the stimulable phosphor sheet for a time corresponding to the amount of erasing energy. For this purpose, the radiation image recording and read-out systems, or the like, are provided with an erasing section, which calculates the amount of erasing energy from the image information having been read out for the reproduction of a radiation image and carries out a noise erasing operation by delivering the calculated amount of erasing energy to the stimulable phosphor sheet.
In the conventional radiation image recording and read-out systems, or the like, when the dose of radiation delivered to the stimulable phosphor sheet is as large as 100 mR or more, particularly 300 mR or more, problems occur due to a saturation phenomenon of the photo detecting surface of the photoelectric conversion means, such as a photomultiplier. This means photoelectrically converts the light emitted by the stimulable phosphor sheet into an electric image signal. Also, ordinarily, the dynamic range of logarithmic conversion, which is carried out after the photoelectric conversion, is limited to approximately 5 orders of ten (10.sup.5). Therefore, it is not always possible to make accurate calculations of a wide range of radiation from a small dose to a large dose, and there is the risk that the linearity of the photoelectric conversion becomes low. As a result, the amount of erasing energy required to erase noise cannot be calculated accurately.